The present invention relates to a planar coil circuit
More particularly, the invention provides an improved method for insulating a face of a planar circuit of the type typically used in a transformer, while leaving the terminals thereof exposed; and to a planar printed circuit or lead frame stamped or etched solid copper similar to printed circuits but with no base material core manufactured using this method.
In long-established practice wound magnetic components such as transformers, solenoids, choke coils, loudspeakers, motors and other magnetic components use multiple coils of round section wires to generate a magnetic field. The round wire carries a thin coat of insulation, and the coil becomes part of a low-cost and reliable component. Power/space efficiency however is not optimum due to the inevitable spaces formed when a plurality of circles or cylinders are brought into contact.
In recent years it has been found that flat magnetic coil components can be produced by the same technology which has long been used for printed circuit boards.
The principal advantage gained by the planar form is that a larger number of coils as a printed circuit and or lead frames can be fitted in to the equivalent space required by round-section wire. The planar printed coil opens up many design options, one of which is that the coil can be of any shape and width, and multiple coils on one face are possible. A wide conductor makes possible high current flow. Weight reduction is another benefit, this being of particular interest in aerospace applications. The planar circuits can be, and in most cases are interconnected with other circuits to generate a magnetic field and to meet a broad array of requirements. Thus a combination of circuits can be used to build a transformer, for example as proposed in U.S. Pat. No. 5,949,321.
Production methods and descriptions of planar circuits are known, see for example U.S. Pat. No. 5,952,909 and No. 6,000,128 to Umeno et al.
Each planar circuit usually needs to be insulated from adjacent circuits and almost always from a ferrite core passing through the planar coil. However the terminals of the circuit need to be exposed so that electrical connections can be attached thereto. The assembly of the circuits is done manually placing insulating material in between two coil circuits. Such assembly is a time and labor consuming operation. In order to overcome the manual assembly operation it has been suggested to insulate the circuits beforehand.
The three known methods of insulating planar circuits are not completely satisfactory.
A liquid solidifying dielectric coating is easy to apply. However the thickness of the coating obtained shows significant variation, particularly in the vicinity of irregular copper shapes printed on the substrate, The coating can also become porous after drying, allowing an electrical discharge when the circuit is in use. In order to meet safety standards, such coatings require testing to conform to standards, and such testing increasing costs.
Conventionally applied polymeric films do not cover the conductors on all sides, and fail to cover the edges of the copper conductors. Application requires skilled workers, and the result is not optimum regarding space utilization.
Bobbins are widely used for supporting coils but the hollow central tube thereof prevents the metallic winding from close proximity to the ferrite core, reducing the efficiency of the magnetic circuit.